For environmental reasons, it is becoming more popular to replace solvent based paints with water bourne paints in many industrial applications. To achieve high coating transfer efficiencies, coatings are often applied with an electrostatic process. As the liquid coating material is atomized, a high electrostatic charge is imparted to the coating droplets relative to a grounded workpiece. The charge on the droplets may be as high as 100,000 volts, or more. The resulting electrostatic field attracts the charged droplets to the workpiece. Even misdirected droplets which would otherwise miss the workpiece are deposited on the workpiece.
Problems occur with electrostatic application of electrically conductive coating materials, such an water bourne paints. When the material supply source is electrically grounded and the material has sufficient conductivity, the material cannot be properly charged with a high voltage electrode exposed to the paint column at the spray gun nozzle. Various techniques have been used for charging electrically conductive paints. One method involves isolating from ground the paint source, the paint supply hoses and the spray gun. As a consequence, all portions of the system exposed to the paint are maintained at the high voltage. Steps must be taken to protect the system operator from exposure to the high voltage. In another type of system, the paint source, the paint hoses and the spray gun are all maintained at ground potential. An electrode is mounted on the spray gun to extend to just outside of the atomized paint envelope adjacent the spray gun nozzle. The electrode is mounted in an insulated holder and has only an exposed high voltage tip.
Although indirect charging apparatus works effectively for charging water bourne paints, its efficiency decreases if paint becomes deposited on the insulated electrode holder. If sufficient paint is deposited on the electrode holder, it may become electrically conductive and grounds the high voltage electrode. U.S. Pat. No. 3,952,951 has reduced this problem by directing jets of air towards the electrode holder. Paint deposits are eliminated where the air jets impinge on the electrode holder. However, the arrangement shown in this patent does not protect all sides of the electrode holder from deposits. Nor does it afford maximum protection at the tip of the holder adjacent the electrode, where risk of paint deposits is greatest because of its close proximity to the atomized paint envelope.